In a process of manufacturing an integrated circuit, there are steps where a thin metallic film is deposited and then partially removed to form metallic conductor wires on a semiconductor substrate.
From the view point of its cost performance and yield, the thickness of the metallic film is preferably as uniform as possible. As one of methods to efficiently obtain such a metallic film, a metal plating technique may be employed.
A conventional plating technique is hereinafter described with reference to FIGS. 1 and 2. Such a metal plating apparatus has been described in "Introducing TAB Technology" by Kenzo Hatada (published by Kogyo Chosakai Publicing Co. on Jan. 25, 1990).
FIG. 1 is a cross-sectional view of a conventional metal electroplating apparatus. As shown in FIG. 1, an overflow cup 1 is a plating tank having an opening on the top and an injection hole 2 on the bottom thereof. The injection hole 2 is connected to an injection line 3 to introduce plating solution 4 into the cup 1. The plating solution 4 flows upwardly and overflows the cup 1. The cup 1 is also provided with a circular mesh-shaped anode 5 therein.
A cathode 6 is capable of absorbing a circular semiconductor substrate 7 on the bottom thereof and of supplying a current to the main surface of the semiconductor substrate 7 to be plated.
The main surface of the semiconductor substrate 7 faces the mesh-shaped anode 5 in parallel with each other. The diameter of the mesh-shaped anode 5 is equal to or greater than that of the semiconductor substrate 7.
Next, the operation of the foregoing arrangement is described. First, the cathode 6 absorbs and fixes the semiconductor substrate 7 and a plating solution 4 is supplied into the cup 1 from an injection hole 2. The injected plating solution 4 flows upwardly through the mesh-shaped anode 5 and overflows the cup 1 while getting in substantially uniform contact with the main surface of the semiconductor substrate 7. In this condition, rotating the semiconductor substrate 7, a current is supplied from the anode 5 to the cathode 6 for electroplating the main surface of the semiconductor substrate 7 with a metallic film.
As shown in FIG. 2, however, the metallic film 8 formed on the main surface of the semiconductor substrate 7 is not uniform in thickness. This is caused by the following phenomena. The current for electroplating flows from the mesh-shaped anode 5 to the main surface of the semiconductor substrate 7, and further flows through the conductor layer of the main surface and the conductor of the outer peripheral portion of the semiconductor substrate 7, and then reaches the cathode 6. Therefore, the nearer a portion of the main surface of the semiconductor substrate 7 lies to the outer peripheral portion thereof, the lower its potential is. On the other hand, the mesh-shaped anode 5 is formed with the same diameter as that of the semiconductor substrate 7 and has a uniformly high potential. Therefore, an electric field 9 generating between the semiconductor substrate 7 and the anode 5 becomes smaller at a portion nearer to the center of the semiconductor substrate 7, while becoming greater at a portion nearer to its outer peripheral portion. Under this condition, since the plating solution 4 is in uniform contact with the whole main surface of the semiconductor substrate 7, the portion which is nearer to the outer peripheral portion attracts more metallic ions so that it may be plated more thickly as shown in FIG. 2.
That is, the conventional plating apparatus has a problem that the metallic film 8 formed on the semiconductor substrate 7 has a nonuniformity of thickness.